Valid palindrome done

Author: rampatra

src/main/java/com/leetcode/strings/ValidPalindrome.java

@@ -0,0 +1,56 @@
+package com.leetcode.strings;
+
+/**
+ * @author rampatra
+ * @since 2019-04-19
+ */
+public class ValidPalindrome {
+
+    /**
+     * Time complexity: O(n)
+     * where,
+     * n = no. of characters in the string
+     * <p>
+     * Runtime: <a href="https://leetcode.com/submissions/detail/223590134/">2 ms on leetcode</a>.
+     *
+     * @param str
+     * @return
+     */
+    private static boolean isPalindrome(String str) {
+        char[] chars = str.toCharArray();
+        int left = 0;
+        int right = chars.length - 1;
+
+        while (left < right) {
+            // if it's not alphanumeric then move the left pointer forward
+            while (!isAlphaNumeric(chars[left]) && left < right) {
+                left++;
+            }
+            // if it's not alphanumeric then move the right pointer backward
+            while (!isAlphaNumeric(chars[right]) && left < right) {
+                right--;
+            }
+
+            // case insensitive comparison
+            if (Character.toLowerCase(chars[left]) != Character.toLowerCase(chars[right])) {
+                return false;
+            }
+
+            left++;
+            right--;
+        }
+
+        return true;
+    }
+
+    private static boolean isAlphaNumeric(char c) {
+        int i = (int) c;
+        return (i >= 48 && i <= 57) || (i >= 65 && i <= 90) || (i >= 97 && i <= 122);
+    }
+
+    public static void main(String[] args) {
+        System.out.println(isPalindrome("A man, a plan, a canal: Panama"));
+        System.out.println(isPalindrome("race a car"));
+        System.out.println(isPalindrome("0P"));
+    }
+}